1. Field of the Invention
The present invention relates to an apparatus and method for providing VoIP service and, more specifically, to an apparatus and method for providing a private network with VoIP service, where an IP terminal included in the private network and an IP terminal included in a public network set a call therebetween and exchange a voice packet according to the VoIP.
2. Description of the Related Art
Currently, an interest in VoIP (Voice over Internet Protocol) service is increasing due to the high spread of the Internet. Such VoIP service provides a phone user with long distance call service or international call service in an Internet environment or an Intranet environment with a local phone service fare, by integrating the phone service using an IP network. And, an SIP (Session Initiation Protocol), an MGCP (Media Gateway Control Protocol), and an H.323 are defined in the VoIP.
The SIP is a simple text-based application-layer control protocol, with which at least one of the participants generates, corrects or terminates a session all together. Such session includes Internet-based video conference, telephone, interview, event notification, and instant messaging.
An MGCP is a standard protocol for signal operation and session management needed in progress of a multimedia conference, which is also known as ‘H.248’ or ‘Megaco’. The H.323 is a standard defined by ‘ITU-T’ in order to transmit data of the multimedia video conference through a network of a packet exchange scheme such as TCP/IP.
Since public IP addresses of the IPv4 address system are gradually exhausted, a method for solving a problem of the insufficient public IP address using NAT (Network Address Translation) or other methods was suggested. Such NAT is used to solve the exhaustion of the public IP address problem by making the IP network use a small number of public IP addresses, and is described in a general agreement of ‘RFC (Request for Comments) 1631’.
The IP network having a general public network includes a private network using a private IP addresses, a public network using a public IP addresses, a network address converter for converting the private IP addresses used in the private network to a corresponding public IP addresses, and a switching system. When a user uses VoIP service through the private IP terminal in the IP network, the private IP terminal generates a call connection request message according to phone number information inputted by the user and transmits the message to the IP network.
The network address converter converts a source IP address in the call connection request message transmitted from the private IP terminal, that is, converts the address from the private IP address to a corresponding public IP address. The switching system identifies an incoming IP address of a call connection request message transmitted from the network address converter, and transmits the call connection request message to the corresponding IP terminal. When the public IP terminal responds to the call connection request message, a call is connected between the private IP terminal and the public IP terminal, and voice communication is performed according to the VoIP.
However, such a method can be used only in the case that a private IP terminal is connected to another private network. That is, when a plurality of private IP terminals or PCs are connected to each other through a private network, the network address converter converts the source IP address of the message transmitted from each of the private IP terminals or PCs into the same public IP address. Since a destination IP address of a packet transmitted from the public IP terminal is a public IP address that is converted by the network address converter, it is not possible to transmit such a packet to a specific private IP terminal or PC. Therefore, it is not possible to provide VoIP service between a public IP terminal and a private IP terminal.
On the other hand, in the case that the public IP terminal transmits a call connection request message to the private IP terminal, the switching system identifies an incoming public IP address transmitted from the public IP terminal, and transmits the call connection request message to the network address converter. When there are a plurality of private IP terminals or PCs that use a single incoming public IP address, the network address converter cannot convert the public IP address to the private IP address since it cannot uniquely determine which specific private IP terminal or PC is to receive the call connection request message transmitted from the public IP terminal.
Accordingly, when the private network having one public IP address is connected to the plurality of private IP terminals, the public IP terminal cannot transmit a call connection request message to an incoming private IP terminal, making it impossible to provide VoIP service between a public IP terminal and a private IP terminal. What is therefore needed is an apparatus and method for providing VoIP service between public IP terminals and private IP terminals.